The present invention relates generally to the field of interconnection structures for joining an integrated circuit to a carrier substrate. In particular, the present invention is related to films for adhering and underfilling soldered integrated circuits to a substrate and methods for doing the same.
Integrated circuit assemblies are utilized in virtually every electronic device. Such assemblies include an integrated circuit (“IC”) and a carrier substrate. Each integrated circuit contains input/output points (“I/Os”), or leads, on its surface. These I/Os are connected to the circuit pattern on the carrier substrate at specific points on the substrate called “lands” or “pads.” So-called “flip-chip” processes are conventionally used for mounting ICs to the carrier substrates.
In a flip-chip process, solder bumps or balls are placed on the I/Os of the integrated circuit. The IC is then inverted and placed directly on the circuitized side of the carrier substrate such that the solder bumps align with the lands on the carrier substrate. Heat is then applied to reflow the solder which forms the solder joint. In the final assembly, a gap remains between the carrier substrate and the IC. Such gap is due to the height of the solder joints.
Although such flip-chip processes offer advantages in the manufacture of electronic devices, such as freeing up enormous space on the carrier substrate and greatly simplifying the manufacture of IC assemblies, such process also has a significant disadvantage. The solder joints in a flip-chip assembly are susceptible to thermal stress and failure. The coefficients of thermal expansion (“CTE”) for the integrated circuit, carrier substrate and the solder joint are all different. Accordingly, these materials swell and contract at substantially different rates as the temperature rises or falls. This subjects the solder joint to tremendous stress, which causes the joints to fail over time.
One approach to solving the CTE stress problem is by filling the gap between the IC and the carrier substrate, such as with a low viscosity polymer composition. Such a process is referred to as “underfilling.” A low viscosity polymer composition is one having a viscosity of 1000 poise or less at room temperature, and 100 poise or less at an application temperature of typically 45° C. or greater. In a conventional underfilling process, the low viscosity polymer composition is dispensed onto two sides of the soldered IC and wicked (i.e. by capillary action) into the gap between the IC and the carrier substrate. The assembly may be heated to increase the rate of flow of the polymer composition into the gap. Once the flow is completed, the underfill composition is applied to the remaining two sides of the IC and the process repeated. The underfill is then typically cured by heating. Underfill materials also provide additional support for the IC and increase the thermal contact between the IC and the carrier substrate.
Conventional underfill materials, which include epoxies and polyimides, have certain disadvantages. For example, certain compositions use anhydride hardening agents which have a tendency to hydrolyze to diacids when subjected to environmental moisture. Such hydrolysis reduces performance. Relatively large thermal expansion differentials still exist in flip-chip assemblies using conventional underfills. This is due to the fact that the cured underfill materials have high CTEs relative to the solder joint.
Periodically, it is necessary to remove the IC from an IC assembly, such as when an IC does not pass inspection. In such cases, underfills that can be re-molded or re-worked are desirable as they facilitate disassembly of the structure. For example, U.S. Pat. No. 6,271,335 (Small et al.) discloses thermally removable encapsulants containing at least one bis(maleimide) compound and a tris- or tetra-furan compound. While such highly cross-linked systems may have superior physical properties, they are often irreversibly damaged (i.e. cracked) by high stresses. Such high stresses arise during continued use of the electronic device containing the IC assembly. It is desirable to have an underfill material that not only solves the aforementioned problems, but is also self-healing. By “self-healing” it is meant that minor defects, such as cracks, in the underfill material are repaired by the material itself. In order to be self-healing, the underfill material needs an amount of flexibility within the polymer and monomers composing the polymer and an amount of reactivity that will allow the underfill material to re-polymerize during use to repair certain defects, such as cracks.
A further problem with conventional underfill materials is that volume changes occur during curing, differential thermal expansion and compositional inhomogeneity can lead to poor interfacial contact, reduction in thermal conductivity and other problems. Underfill materials are therefore desired that do not substantially change volume during curing.